The invention is in the field of production of crystalline materials, and in particular to phonon-enhanced crystal growth and lattice healing.
Modern semiconductor technology demands high-quality crystalline solids for high-performance and high-efficiency devices. In the case of solar cells (PV, photovoltaic), where the materials quality demands are less stringent, half of the industry has adopted a multicrystalline silicon (mc-Si) based technology. While manufacturing costs of mc-Si are lower than single-crystalline silicon, the efficiencies achievable with mc-Si materials are also significantly lower. The principal performance limitation of mc-Si lies in its high defect content. Dislocations, grain boundaries and impurities are the principal causes of carrier recombination and lifetime losses. The majority of these defects arise from the use of a less pure silicon feedstock source and a less sophisticated crystal growth technique.
During the growth of silicon crystals, the thermal characteristics of the growing system are the most important consideration. The morphology, crystal quality, and impurity content are determined by the nature of the heat flow balance around the melt-crystal interfaces.
In the case of ribbon or foil mc-Si, the crystal cools by radiation and gas convection at the crystal surface, while the melt radial temperature gradient is determined by the melt aspect ratio and heater geometry. In the case of ingot mc-Si, the crystal cools by conduction through the crucible walls, as well as radiatively and convectively via the top surface.
The interface shape and isotherms in the growing crystal are controlled by the melt temperature gradients and the crystal cooling conditions as well as the crystallization rates.
These configurations correspond to high thermal stress conditions, which can generate dislocations and small-angle grain boundaries when the thermal stress exceeds the critical resolved shear stress.
Specifically, structural defects like dislocations play an important role in lifetime degradation since they act as centers for recombination of charge carriers (electrons, holes). It is known that a dislocation density decrease from 106 cm−2 to 104 cm−2 results in a lifetime increase of 70%. Likewise, the reduction of dislocation densities in mc-Si from 107 cm−2 (found in as-grown wafers) to <104 cm−2 can potentially increase photovoltaic efficiencies on the order of 10-20% relative. While this is especially promising in photovoltaics, where mc-Si is the major starting material, removal of dislocations with low manufacturing cost would have a tremendous impact in other industries such as semiconductor device processing.
It has been shown that high temperatures and stresses enhance dislocation motion and help alleviate the crystallographic defects found in different sources of mc-Si (e.g. string-ribbon, ingot mc-Si). Previous studies also show that by applying mechanical loads and high temperature to solid blocks/bricks and wafers of multicrystalline silicon, dislocation densities can be reduced.